1. Field of the Invention
This invention relates to the art of electrophotography and more particularly, to photosensitive materials for electrophotography which make use of organic photosensitive compounds and are particularly suitable for use in electrophotography for positive charge systems.
2. Description of the Prior Art
Extensive studies and developments have now been made on organic photosensitive substances or compounds. The organic photosensitive compounds have a number of advantages over inorganic photosensitive compounds, including the ease in preparation of a variety of compounds exhibiting high sensitivity at different wavelengths depending on the molecular design, little or no ecological problem, good productivity and economy, and inexpensiveness. Although the problems hitherto involved in organic photosensitive compounds include durability and sensitivity, these characteristic properties have been remarkably improved at present. Some organic photosensitive compounds have now been in use mainly as photosensitive materials for electrophotography.
Known organic photosensitive materials usually have a double-layer structure which includes a charge generating layer capable of absorbing light to generate carriers and a charge transfer layer wherein the generated carriers are transferred. Many attempts have been made to make photosensitive materials with high sensitivity. Known materials used to form the charge generating layer include perylene compounds, various phthalocyanine compounds, this pyrylium compounds, anthanthrone compounds, squalilium compounds, bisazo compounds, trisazo pigments, azulenium compounds and the like.
On the other hand, the materials used to form the charge transfer layer include various types of hydrazone compounds, oxazole compounds, triphenylmethane compounds, arylamine compounds and the like.
In recent years, there is a high demand of photosensitive materials for digital recording such as in laser printers wherein the organic photosensitive compounds indicated above are used in a near ultraviolet range corresponding to semiconductive laser beams with a wavelength range of from 780 to 830 nm. Accordingly, organic photosensitive compounds having high sensitivity in the above-indicated near ultraviolet range have been extensively studied and developed. In view of the sensitivity in the above UV range, organic photosensitive compounds are more advantageous than inorganic photosensitive compounds or materials.
The organic photosensitive compounds are usually employed in combination with binder resins and applied onto substrates, such as a drum, a belt and the like, by relatively simple coating techniques. Examples of the binder resins used for this purpose include polyester resins, polycarbonate resins, acrylic resins, acryl-styrene resins and the like. In general, with the double-layer structure, the charge generating layer is applied in a thickness of several micrometers in order to attain high sensitivity and the charge transfer layer is applied in a thickness of several tens of micrometers. From the standpoint of the physical strength and the printing resistance, the charge generating layer should generally be formed directly on the substrate and the charge transfer layer is formed as a surface layer. In this arrangement, charge transfer compounds which are now in use are only those which act by movement of positive holes. Thus, the known photosensitive materials of the double-layer structure are of the negative charge type.
The negative charge systems, however, have several disadvantages: (1) negative charges used for charging attack oxygen in air into ozone; (2) charging does not proceed satisfactorily; (3) the system is apt to be influenced by surface properties of a substrate such as a drum. Ozone presents the problem that not only ozone is harmful to human beings, but also it often reacts with organic photosensitive compounds to shorten the life of the photosensitive materials. The instability of the charging often invites a lowering of image quality. The influences of the surface properties requires a mirror finish on the substrate surface, thus needing an undercoating on the surface. This leads to an additional production cost. The known double-layer photosensitive materials have further disadvantages: (4) the fabrication process becomes complicated; and (5) the stability is not satisfactory because of the separation between the layers.
In order to solve the above problems, organic photosensitive materials of the positive charge type have been extensively studied. In order to attain the positive charge systems, attempts have been heretofore made including (1) reversed double-layer structures wherein the charge generating layer and the charge transfer layer are reversed to the case of the negative charge type; (2) single-layer structures wherein various types of charge generating compounds and charge transfer compounds are dispersed in binder resins; and (3) a single-layer structure wherein copper phthalocyanine is dispersed in polymers.
However, the reversed double-layer structure involves the problems similar to the negative charge system, i.e. complicated fabrication processes and the separation of the two layers. In addition, the charge generating layer, which has to be substantially thin, is placed on the surface of the photosensitive material with attendant problems such as reduction in the printing resistance and a poor life characteristic.
On the other hand, the photosensitive materials having the single-layer structure as in (2) and (3) above which are of the positive charging type are inferior to the double-layer structure photosensitive materials with respect to the sensitivity, charging characteristics, i.e. the materials are less likely to be charged, and a great residual potential. The reason why the sensitivity is poorer is that the generation and transfer of charges take place randomly in the single layer. Thus, the photosensitive materials having the single-layer structure has the problem to solve when used in practical applications.
As will be appreciated from the above, the known organic photosensitive materials have some problems to solve.